1. Field of the Invention
The invention relates to an adjustable length column for chairs and tables, comprising an upright tube and a pneumatic or hydropneumatic length adjusting element, which is disposed therein concentrically of a common central longitudinal axis, and the housing of which has an outer surface, which is guided for displacement in the direction of the axis and radially supported in a guide bush which is disposed in the upright tube.
2. Background Art
Adjustable length columns of the generic type are known from U.S. Pat. No. 4,969,619. In these adjustable length columns and the length adjusting elements employed therein, which are regularly adjustable length gas springs, the surface, i.e. the outside wall of the housing of the length adjusting element, is polished and then chromium plated or browned, polishing being necessary for impeccable guidance in the guide bush of the upright tube to be ensured. In particular, the surface roughness of the outside wall of the housing must be very low so that in the guide bush, which regularly consists of plastic material, no unnecessary wear will occur, as a result of which the radial zero backlash would be lost. In particular with browned surfaces, it has been found that the browning is affected in the course of time by the friction in the guide bush.
It is an object of the invention to embody an adjustable length column of the generic type and a length adjusting element therefor of the generic type such that the lifetime of the browning of the surface of the outside wall of the length adjusting element is increased.
According to the invention, this object is achieved in an adjustable length column of the generic type by the outer surface of the housing being unpolished, shot peened and having a browned layer. Furthermore, this object is achieved by a length adjusting element for such a column.
The design of the length adjusting element according to the invention not only helps attain that the expensive polishing of the surface of the outside wall of the housing is replaced by less complicated shot peening, but it also ensures that the browning applied after shot peening will be more durable. The reason may reside in that the surface obtains an extremely finely grained structure by the browning, only the upper edges of the minute depressions produced by shot peening taking their beating on the guide bush and the browning being rubbed off only there in the course of time, whereas it is maintained in the depressions. Thus, it is optically maintained for a prolonged time.
When shot peening is carried out with special intensity, then ribs may form on the edges of the grains, these ribs being slightly too marked. In these cases, it can be of advantage that the surface is burnished on the browned layer, i.e. these ribs are slightly smoothed by means of so-called burnishing rollers, which are guided under pressure along the surface.